In recent years, motors driven by the inverter of PWM (Pulse Width Modulation) method have been widely used in the market. In the case of the motors driven by the PWM inverters, an electric potential at the neutral point of the winding cannot be 0 (zero), so that an electric potential difference (hereinafter referred to as an axial voltage) is generated between the outer ring and the inner ring of the bearing. The axial voltage contains a high-frequency signal produced by the switching operation. When the axial voltage reaches a dielectric breakdown voltage of the oil film inside the bearing, a micro electric current (hereinafter referred to as an axial current) runs in the bearing, thereby causing electric erosion in the bearing. Development of electric erosion will result in wavy abrasions on the inner ring, the outer ring, or the balls of the bearing and the wavy abrasions sometimes cause abnormal sound. The electric erosion is thus one of chief factors causing defects of the motor.
The following measures have been taken for preventing the electric erosion:
(1) The inner ring and the outer ring of the bearing are made conductive to each other.
(2) The axial voltage is lowered.
(3) The inner ring is insulated from the outer ring of the bearing.
Measure (1) employs, e.g., a conductive lubricant for the bearing. However, the conductive lubricant loses the conductivity over time, and has an issue of insufficient lubrication. A brush can be mounted on the rotary shaft for making the inner ring and the outer ring conductive to each other; however, this measure cannot be implemented without abrasion dust of the brush and requires a space for installing the brush.
Measure (2), e.g., electrically shorts the stator iron-core to a conductive metallic bracket, thereby varying the electrostatic capacity and lowering the axial voltage. This measure is well known and disclosed in, e.g., Patent Literature 1.
Patent Literature 1 discloses that the stator iron-core is electrically shorted to the bracket for lowering impedance on the stator side, thereby suppressing the electric erosion on the bearing.
To be more specific, the motors used in devices operated in a wet area, e.g. washing machine and dish washer, generally have a risk of causing an electric shock, so that not only is an insulation needed on a charging section (primary insulation), but also an additional insulation independently from the primary insulation (hereinafter referred to as an additional insulation) is needed. On the other hand, motors used in other appliances, e.g., an indoor unit or an outdoor unit of an air-conditioner, a water heater, or an air-cleaner, have no risk of causing an electric shock, so that the additional insulation is not needed. Therefore, the motors used in the indoor unit or outdoor unit of the air-conditioner, the water heater, or the air-cleaner thus employ rotors made not insulated, so that the impedance on the rotor side (the inner ring side of the bearing) stays low, while the impedance on the stator side (the outer ring side of the bearing) stays high. Accordingly, the electric potential on the inner ring side is high while that on the outer ring side is low, so that the potentials are unequal, and a high axial voltage is generated. This high axial voltage may cause electric erosion on the bearing.
In order to avoid the foregoing problem, Patent Literature 1 proposes a method of lowering the impedance on the stator side (the outer ring side) by electrically shorting the stator iron-core to the bracket and thereby eliminating the electrostatic capacity component between the stator iron-core and the bracket. As a result, the impedance on the stator side becomes close to the impedance on the rotor side (the inner ring side), and the difference in electric potential between the inner ring and the outer ring of the bearing, i.e. the axial voltage, is lowered.
However, the method disclosed by Patent Literature 1 lowers the impedance, so that the voltage drop becomes smaller, and the voltage on the outer ring side as well as the voltage of the inner ring side increase. If the impedance balance is lost due to operating environments of the motor and inaccuracies in assembling the stator and the rotor, the axial voltage tends to rise and cause electric corrosion contrary to the expectation.
Measure (3) conventionally employs replacing all the iron-balls in the bearing with non-conductive balls made of electrically insulating material, such as ceramic. See Patent Literature 2. However, this measure is expensive to implement, although a high anti-erosion effect can be expected.
In recent years, as Patent Literature 1 discloses, a molded motor has been proposed in which a fixing member provided on the stator side, such as the stator iron-core, is integrally molded by a mold member to improve the reliability. Further, the molded motor may be of a simpler structure achieved by using a resin housing, which is a part of the mold member, to fix the bearing. On the other hand, given the structural strength necessary to fix the bearing, a metal bracket provides a structurally stronger support for the bearing than the member. For this reason, in these days, the bearings are fixed selectively with a metal bracket or a mold member accordingly to the structural strength required to motors. To be more specific, molded motors have been proposed in which the bearing on the counter output shaft side, where a high fixing strength is not required, is fixed with a part of the mold member of the integral molding, and the bearing on the output shaft side, where a high fixing strength is required, is fixed with a metal bracket.
However, the motor normally uses two bearings to support the shaft, and the electric corrosion problem will arise for the following reasons it as discussed above, one bearing is fixed with a metal bracket and the other bearing is fixed with a resin housing. The resin housing is highly insulative while the metal bracket is conductive. Therefore, limited axial current flows between the inner ring and the outer ring of the hearing fixed with the resin housing, while the axial current relatively freely flows between the inner and outer rings of the bearing fixed with the conductive metal bracket which lowers the insulability. As a result, electric erosion tends to occur on the bearing fixed with the bracket. In other words, the problem arises that electric corrosion occurs mainly on one of the bearing when the two bearings are fixed with fixing members of different materials. Further, even if the two bearings are fixed with metal brackets, their insulabilities may not be equal depending on their sizes and places where the brackets are placed, resulting in that electric erosion tends to concentrate on one bearing. In such a case, the service life of the motor becomes as short as the service life of the one bearing.    Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2007-159302    Patent Literature 2: Unexamined Japanese Patent Application Publication No. 2007-16846